1. Field of the Invention
The present invention relates to an inner-rotor motor. In particular, the present invention relates to an improvement in a motor in which a stator is arranged radially outside of and opposite to a rotor magnet.
2. Description of the Related Art
In a common inner-rotor motor, a rotor magnet is fixed to a shaft, and a stator is arranged radially outside the rotor magnet. In addition, bearings arranged to support the shaft are arranged on an upper side and a lower side of the rotor magnet, and each of the bearings is supported by a bracket arranged to cover the stator. The shape of the stator depends on desired characteristics of the motor, such as power and torque. Therefore, different brackets should be used for different motors having different desired characteristics, and it is impossible to use a common bracket for the different motors to achieve a reduction in costs of the motors.
Meanwhile, a motor is known in which both ends of a stator are held by two brackets, and these brackets are fixed and fastened by a fixing screw (see JP-A 11-220860, for example). Adoption of a structure described in JP-A 11-220860 makes it possible to use the same brackets for different motors whose stators have different axial dimensions.
However, the motor described in JP-A 11-220860 has a problem in that use of the fixing screw is necessary to fix and fasten the brackets, and the number of parts is increased. In addition, the motor described in JP-A 11-220860 has a problem in that fixing screws having different lengths should be used in accordance with the axial dimension of the stator. Furthermore, the motor described in JP-A 11-220860 has a problem in that a space in which the fixing screw is inserted should be secured in the stator, and this leads to a deterioration in the magnetic property of a stator core and a decrease in an energy efficiency of the motor.
An interspace between the brackets depends on the axial dimension of the stator, and is affected by an error in the shape of the stator. As a result, in the case where a sleeve bearing is used to support the shaft, a groove to which a stopper washer is fixed is often defined in the shaft, and the error in the shape of the stator affects a gap between the stopper washer and the sleeve bearing. As a result, play between the stopper washer and the sleeve bearing is changed. That is, in the case where axial play between the stopper washer and the sleeve bearing is too small, a shaft loss may be increased. Meanwhile, in the case where the axial play between the stopper washer and the sleeve bearing is too great, a rotor may become shaky.